2.6.27-stable review patch. If anyone has any objections, please let us know.

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From: Li Zefan

commit 24eb089950ce44603b30a3145a2c8520e2b55bb1 upstream

This fixes an oops when reading /proc/sched_debug.

A cgroup won't be removed completely until finishing cgroup_diput(), so we
shouldn't invalidate cgrp->dentry in cgroup_rmdir(). Otherwise, when a
group is being removed while cgroup_path() gets called, we may trigger
NULL dereference BUG.